'Robes' in the Bible

After these things I looked, and here was an enormous crowd that no one could count, made up of persons from every nation, tribe, people, and language, standing before the throne and before the Lamb dressed in long white robes, and with palm branches in their hands.

Then one of the elders asked me, "These dressed in long white robes -- who are they and where have they come from?"

So I said to him, "My lord, you know the answer." Then he said to me, "These are the ones who have come out of the great tribulation. They have washed their robes and made them white in the blood of the Lamb!

Blessed are those who wash their robes so they can have access to the tree of life and can enter into the city by the gates.